This invention relates generally to fluid delivery systems and more particularly to valve assemblies that must handle particulate-containing fluids.
It is common to pump fluids that contain particulates into oil and gas wells. For example, fracturing fluids typically contain proppant particles, such as sand or small beads, (sizes typically from U.S. Standard Sieve sizes 10 through 60). Reciprocating plunger pumps are frequently used to create the high-pressure fluid flow needed to inject fluids, such as fracturing fluids, into oil and gas formations. These pumps typically include valve assemblies that are biased toward the closed position. When the motion of the plunger creates a differential pressure across the valve, the differential pressure forces the valve open, allowing the fluid to flow through the valve. However, solid particles in the fluid can become trapped within the valve assembly upon valve closure, creating damage to valve assembly components and reducing the useful life of the valve assembly.
The valve assembly will typically contain an area where two metal surfaces contact each other when the valve is closed. The solid particles from the fluid can become trapped between the two metal contact surfaces in specific locations rather than evenly distributed across those surfaces, creating concentrated stress forces at these locations. These concentrated stress forces can lead to localized pitting. Once pitting has occurred, the solid particles tend to concentrate at the location of the pitting, which in turn accelerates the damage at these locations.
Valves used for slurry service typically have a resilient sealing insert around the outer perimeter of the valve closure member to provide effective valve sealing. Pressure applied to a closed valve forces the resilient sealing insert to become a hydraulic seal, extruded into the gap between the valve closure member and the valve seat member. For the insert to effect a hydraulic seal upon valve closure, the insert must protrude from the valve closure member toward the valve seat member when the valve is open. When the valve is nearly closed, the resilient sealing insert contacts the valve seat member. When the valve is closed, the resilient sealing insert is deformed against the seat member to form the hydraulic seal, and metal-to-metal contact occurs between the valve closure member and the valve seat member. Proppant trapped under the resilient sealing insert can become temporarily or permanently embedded in the resilient insert material, so that the insert can effect a hydraulic seal in the presence of proppant. In the presence of proppant, the metal surfaces of the valve closure member and valve seat member do not form a hydraulic seal.
The resilient sealing insert of current valves is on the outer perimeter of the valve closure member or valve seat member, so that applied pressure will deform the resilient sealing insert to seal between the valve closure member and the valve seat member. If the resilient sealing insert were on the inner perimeter of the valve closure member or valve seat member, then applied pressure would force the resilient sealing insert away from the contact area between the valve closure member and the valve seat member, and the valve would not seal.
The resilient sealing insert of current valves contacts the valve seat member before the valve closure member contacts the valve seat member. The gap between the sealing insert and the seat of an open valve is smaller than the gap between the valve closure member and the valve seat. When the valve is closing, the gap between the sealing insert and the valve seat member becomes too small to pass particles in the fluid, while the gap between the valve closure member and the valve seat member is still large enough to pass particles into the region between them. Thus a standard valve sealing insert can act as a forward screening element that concentrates proppant particles in the region between the valve closure member and the valve seat member. Such concentrations of proppant particles cause damage to the contacting surfaces of the valve closure member and the valve seat member.
If the pump is operated in such a way as to have significant valve lag, i.e. a discharge valve does not close until well after the plunger starts its suction stroke, there will be reverse flow through the valve before it closes. The standard sealing insert will screen out proppant particles from the reverse fluid flow, preventing the particles from entering the region between the valve closure member and the valve seat member. However, the volume of fluid which flows through current valves during the short time interval between the onset of such reverse particle screening and the closure of the valve typically is insufficient to displace the proppant-laden fluid from the valve before closure. Particles are still trapped between the valve closure member and the valve seat member.
Conventional liquid end valve assemblies may also experience failures due to foreign objects becoming lodged within the valve assembly (e.g., bolts or gravel can accidentally enter the fluid flow path). These foreign objects can become wedged between the contact surfaces of the valve, and thus prevent the valve from closing.
There is a need for improved valve assemblies that reduce the incidence of damage caused by particulates or foreign objects in well treating fluids.